It is known to actuate a closure member such as a trunk lid by using a power actuator. See Japanese patent laid open publication No. 2002-180738. Such a power actuator system typically comprises an electric motor, a reduction gear unit and a link mechanism. The base end of a first link is fixedly attached to the output shaft of the reduction gear unit, and the free end of the link is connected to the base end of a second link. The free end of the second link is connected to a hinge arm which is in turn fixedly attached to a hinge end of the trunk lid. A damper that resiliently urges the trunk lid in the opening direction is connected between the hinge arm and a part of the vehicle body. By turning the output shaft in each direction, the trunk lid can be opened and closed at will.
In such an arrangement, the load acting upon the power actuator is primarily dictated by the weight of the trunk lid and the thrust of the damper, and varies significantly depending on the angular position of the trunk lid. When the trunk lid is fully closed and is substantially horizontal, although the damper produces a maximum force, the weight of the trunk lid is so dominant that a relatively large torque is required for the actuator to raise the trunk lid from the fully closed position. As the opening angle of the trunk lid increases, the effect of the weight diminishes while the thrust of the damper in the direction to open the trunk lid becomes more pronounced so that a relatively small torque is required for the actuator to further open the trunk lid.
Conversely, when closing the trunk lid from the fully open state, the thrust of the damper is at a minimum value and the load of the weight of the trunk lid acting in the closing direction is also at a minimum because the trunk lid is at a substantially upright position so that the actuator is only required to overcome the small thrust of the damper. As the trunk lid 1 moves away from the fully closed position, the load acting in the closing direction progressively increases, and a relatively small torque is required to close the trunk lid. When the trunk lid is about to be fully closed, the thrust of the damper acting in the open direction is at a maximum and the reaction force of the weather strip is required to be overcome. Therefore, a substantial torque is required for the actuator to fully close the trunk lid and engages the latch against the resistance of the weather strip.
In such a conventional actuator for a trunk lid, the thrust of the damper is selected in such a manner that the power actuator is required only when moving the trunk lid from the fully closed state to a slightly open state, and the damper provides a force required to move the trunk lid from the slightly open state to the fully open state. Thereby, the torque requirement of the power actuator is minimized, and the power actuator may be designed as a highly compact unit. However, in such an arrangement, it is necessary to adjust the torque output of the power actuator depending on the opening angle of the trunk lid, and this requires a highly complex control arrangement. In particular, it is necessary to provide an angle sensor for detecting the opening angle of the power actuator, and this increases the cost.
When the trunk lid is to be actuated by a power actuator from the fully closed state to the fully open state, no complex control is required, but the power actuator is required to have a relatively large output and this undesirably increases the size of the power actuator. Because the power actuator of this type is required be installed in the limited space of the trunk, the power actuator is required to be as small as possible and any protrusion into the trunk room is desired to be minimized.